Why do you think that you need this feature?

What keeps you from checking the size of @_ in the subroutine?

sub iWantFourArguments
{
  @_==4 or die "Call me with four arguments. Died";
  # ...
}
[download]

Note that prototypes won't help you here, you can bypass them using &subname(...) and they aren't checked at all when subs are invoked as methods.

Alexander